In the manufacture of composite material articles, the fibrous preform must be prepared in such a manner as to confer the desired mechanical properties on the articles.
The fibrous preform is generally obtained from one-dimensional elements, such as yarns or cables, or two-dimensional textures such as cloth, felt, . . . .
Yarns or cables are wound to form axially-symmetrical wound preforms or they are laid parallel to one another in one-directional sheets. The sheets are superposed while ensuring that the directions of the yarns or cables in two adjacent sheets are different.
Cloth or felt is rolled up onto itself or is cut out into plies that are superposed so as to build up the desired thickness for the preform.
In order to improve the mechanical properties of composite materials in which the preform is made up of plies or sheets that are rolled up or superposed, proposals have been made to bond the plies or sheets together by means of yarns or fibers extending transversely thereto. This can avoid the phenomenon of delamination, i.e. of the material splitting between sheets or plies.
Needling is a technique for bonding together plies or sheets that is known and relatively easy to implement.
One difficulty encountered with known refractory fibers resides in their poor ability to undergo textile forming operation, such as weaving, notably in the case of ceramic fibers, and especially as regards needling.
A known way of overcoming this difficulty consists in conducting all the necessary textile-forming operations on yarns whose constituent fibers are in the precursor state, where they are more apt to undergo these operations. The transformation of the precursor into a refractory material is then performed after carrying the textile operations.
Another known way of overcoming this difficulty, when needling superposed plies of a carbon fiber cloth, or superposed unidirectional sheets made of carbon fibers, consists in interposing layers of felt between the plies or sheets. When using a cloth or a sheet formed from yarns in which the cohesion of the fibers is ensured by twisting, the penetration of the yarns by the needling action has more the effect of breaking the fibers than detaching the fibers to allow implantation across the plies. Accordingly, the interposed felt layers are provided to serve as a source of fibers capable of being drawn along by the needling action.
A further problem encountered in the manufacture of composite material articles concerns the accessibility of the internal pores of the fibrous preform during densification.
Different densification techniques are known, such as resin densification and chemical vapor deposition or infiltration.
Resin densification consists in impregnating the preform with a liquid containing a precursor of the material forming the matrix and then transforming the precursor, usually through a heat treatment. Usually the precursor is a polymer which is cured and pyrolysed to obtain the matrix material. The process including impregnation, curing and pyrolysis may be carried out several times.
Chemical vapor deposition or infiltration involves placing the preform in an enclosure into which a gaseous flow is introduced under predetermined temperature and pressure conditions. The gaseous flow thus forms the matrix material upon contact with the fibers of the preform, though a decomposition of one or several its constituents, or by a reaction between its constituents.
Whatever the technique used, it is impossible in practice to achieve a complete densification of the preform. The reason is that some of the volumes that the yarns define between themselves include "dead" volumes. These "dead" volumes cannot be densified, even if a chemical vapor infiltration process is used, their restricted access, if at all present, becoming rapidly obstructed.